OSPF DR BDR Election Explained

OSPF DR BDR Election Explained

OSPF DR/BDR Election Explained

In a broadcast network there is an election process that takes place to elect the DR (Designated Router) and the BDR (Backup Designated Router)

This multi-access network looks like below. See below diagram, 4 routers connected to a Switch.

4 routers connected to a Switch

In this scenario, if the devices are connected as fully meshed, the number of adjacencies increases. The below formula can be used for calculation.

n(n-1)/2 , where n is number of routers

Thus, it would cause unwanted LSA traffic.  The solution to this issue is the DR and BDR.

In a Designated Router, the multicast address 224.0.0.6 can be accessed via the multicast address. A Backup Designated Router plays an important role and is used for redundancy.

The information by other routers in the broadcast network is only sent to the DR and BDR and in trunk they send it to other routers, thus reducing OSPF traffic. Any change in the network is only share with the Designated Router which then propagates the update to other routers.

The multicast IP address 224.0.0.5 is used by the DR to send routing updates. The BDR take over the role in case of any failure with the designated router.

NOTE: DR and BDR are not elected on Point to point links as only 2 routers are used.

In the election these points need to be considered.

  1. Highest Priority Router becomes the DR but by default the priority is 1 for all the routers.

 

  1. In case of a tie, the election is won by the router having the highest Router ID.

 

  1. The “show ip ospf neighbor” command will show if the router is a DR or BDR t any other router.

The default priority is 1 but the priority can be changed by

Router(config-if)# ip ospf priority <priority number>

In case you do not want a router to participate in the DR / BDR election, then its priority must be set as 0.

The router with the second highest priority or the second highest Router ID becomes the BDR.

Note: Preemption is not supported in DR/ BDR which means, if in an existing network where DR and BDR are already elected and you add a new Router with a lower Router / priority ID, the new Router cannot be the DR or BDR till the existing DR / BDR fail and the election happens again.

To learn OSPF in details, please look at our complete step by step OSPF configuration tutorial

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